What is a note?
Play a note on anything, a synth, a guitar, your own voice, and somewhere a surface is pushing the air back and forth very fast. Push it back and forth 220 times in one second and you hear one particular pitch. A note is a number: how many times per second the air moves. Everything else in music theory is built on top of that number.
Dial a note
Turn your volume down a touch, then drag the slider and press Hear it. The green line is the air pressure at your ear, slowed about 100 times so your eye can follow what your ear is doing.
Twist some knobs!
Drag right and two things happen together: the wave squeezes tighter, and the sound gets higher. That is the entire relationship. **Faster is higher.** There is nothing else hiding underneath; a "high" note is just air moving back and forth more times per second than a "low" one.
Naming the number
You have heard it, so now you can name it. The number of back-and-forths per second is called frequency. Its unit is hertz (Hz): 220 Hz means 220 back-and-forths every second. pitch is the musical word for how high or low a frequency sounds. The slider runs from 55 Hz, where basslines live, to 880 Hz, where leads and vocals sit.
The demo names a "nearest note" because the slider is smooth and notes are not. Every frequency between 55 and 880 exists, and you can play any of them; 236.4 Hz is a perfectly real sound. A note is simply a frequency that got picked: one of a small set we all agreed on, so that instruments can play together. Which frequencies got picked, and why there are twelve per octave, is its own story (the short version is in The math behind music).
Before you leave the slider, try this: park it at 110, then 220, then 440, then 880. Each doubling lands on a note that feels like the same note, only higher, and every one of them is named A. That doubling distance is called an octave, and why doubling works that way is the next article on this trail.
In your music
Every note in a Live clip is one of these numbers wearing a name badge. A row in the piano roll is a frequency; the note called A2 is the air moving 220 times a second, whatever instrument plays it. When this site or the Theory Aide extension talks about intervals, chords, or keys, it is doing arithmetic on exactly these numbers, and nothing more mysterious than that is ever going on.